Advance of electrical equipment represented by motors or transformers has been progressed resulting in size reduction and improved performance during recent years. Thus, it becomes usual in many cases that insulated wires are used in such a way that the insulated wires are processed by winding (also referred to as coil processing) to winding wires and they are pushed into a quite small space to pack. Specifically, it is no exaggeration to say that the performance of a rotating electric machine, such as a motor, is determined by how many wound wires produced by coil processing the insulated wires into a coil shape can be held in a stator slot. As a result, improvement of the ratio of the sectional area of conductors to the sectional area of the stator slot (space factor) has been required to be particularly highly increased.
As a means of improving the space factor, a rectangular conductor whose cross-section is an approximate rectangular shape (hereinafter, referred to simply as “a rectangular conductor” or “a conductor”) has been adopted. In a case of producing a small coil by using a rectangular conductor, in order to improve the space factor, the small coil is produced by an edge-wise-winding processing of an insulated wire having a small radius of curvature at a corner of the conductor, around a small diameter of a core. In such edge-wise coil, the load on the insulated wire is very large. For this reason, further advancement of mechanical characteristics of the insulated wire is required.
As a means of advancing the mechanical characteristics of the insulated wire, an attempt to improve an adhesion property between a conductor and an insulating coating has been made. For example, Patent Literature 1 discloses an insulated wire whose dynamic properties have been improved by forming an insulating coating on a previously heated conductor thereby to increase an adhesion property of the insulating coating to the conductor. Further, Patent Literature 2 discloses a covered rectangular wire in which an adhesion property between a coating material and a conductor has been improved by providing a viscoelastic body layer on a substrate surface.